The present invention relates to a method for forming a protective coating on a turbine engine component. More particularly, the invention relates to forming an aluminide or chromide coating on the surface of a turbine engine rotor component, such as a compressor or turbine disk, seal element or shaft. The coated component is typically then heated or maintained at an elevated temperature in the presence of oxygen to form a protective oxide coating on the surface of the component. The invention also relates to such a coated rotor component.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. Thus, there is incentive to raise the combustion gas temperature.
The turbine engine includes compressor and turbine disks (sometimes termed compressor and turbine rotors), a number of blades mounted to the compressor and turbine disks and extending radially outwardly therefrom into the gas flow path, and rotating seal elements that channel the airflow and prevent the hot combustion gases from contacting the turbine shaft and related components. As the maximum operating temperature of the turbine engine increases, the compressor and turbine disks and seal elements are subjected to higher temperatures. As a result, oxidation and corrosion of the disks and seal elements have become of greater concern. Alkaline sulfate deposits resulting from ingested dirt and sulfur in the combustion gas are a major source of the corrosion, but other elements in the aggressive combustion and bleed gas environment may also accelerate the corrosion. The oxidation and corrosion damage may lead to premature removal and replacement of the disks and seal elements unless the damage is reduced or repaired.
Turbine and compressor disks and seal elements for use at the highest operating temperatures are made of nickel-base superalloys selected for good elevated temperature toughness and fatigue resistance. These superalloys are selected for their mechanical properties. They have resistance to oxidation and corrosion damage, but that resistance is not sufficient to protect them at the operating temperatures now being reached. Disks and other rotor components made from newer generation alloys may also contain lower levels of aluminum and chromium, and may be more susceptible to corrosion attack.
Turbine and compressor disks and seal elements typically have not been coated to protect them against oxidation and corrosion. A number of oxidation-resistant and corrosion-resistant coatings have been considered for use on turbine blades. These turbine blade coatings are generally too thick and heavy for use on disks and seal elements, and also may adversely affect the fatigue life of the disks and seal elements. There remains a need for protecting disks, seal elements, and other rotor components against oxidation and corrosion as their operating temperatures increase.